- how do you transform a squad into Berserker ?
- why don't army regain Health simply when garrison and you have to bring your troops to the pool and wait ?
- how can you bring a laborer in your army, to loot food... Seems that you just can't choose the type of troops you want to include in your army because they always appear in the same order...

First you take some Laborers or Marauders and double right click with them on the War Hall or Pyre after what they will be transformed in to Slayers or Impalers. Now using those units the same way on a Temple You can have Berserkers.

- how can you bring a laborer in your army, to loot food... Seems that you just can't choose the type of troops you want to include in your army because they always appear in the same order...

When creating an army, you can choose your units not only by order but by category. See there should be some icons ower youre units and below resources on army transfer/create screen/tab.